Handheld radar frequency scanner for concealed object detection

ABSTRACT

A handheld radar frequency scanner system for detecting objects concealed on a person, the system includes a radar transmitter coupled to a transmit antenna that is configured and positioned to direct a radar signal at a person, a radar receiver coupled to a receive antenna that is configured to detect a portion of the radar signal reflected by the person, and a processor connected to the radar receiver and operable to process the portions of the radar signals detected by the radar receiver to determine whether the person is carrying a concealed object. The system may produce a real-time alert, such as an audible alert, when a concealed object is detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from provisional application No.60/334,533, filed Dec. 3, 2001, which is incorporated by reference.

TECHNICAL FIELD

This description relates to a handheld scanner for detecting concealedobjects, such as objects concealed on a person who is entering a securedarea.

BACKGROUND

One common approach to detecting concealed objects on a person is toscan the person using a handheld metal detector at a securitycheckpoint.

SUMMARY

Techniques are described for implementations of a handheld scanner thatemploys radar frequency detection algorithms to yield a very highprobability of detection against concealed objects that can be dangerousor undesirable. The high detection probability is coupled with a lowfalse alarm rate. In particular, a handheld radar frequency scanner mayemploy target detection technology that uses radar signals in anaviation security context in order to counteract potential terroristacts involving smuggling of explosives and/or other objects on the body.The handheld radar frequency scanner detects differences in reflectedradar frequency energy due to an object placed between the clothing andthe skin of a person.

In one implementation, a handheld radar frequency scanner for detectionof objects concealed on a person includes a radar transmitter andtransmit antenna that are configured and positioned to direct a radarsignal at a person, and a radar receiver and receive antenna that areconfigured and positioned to detect a portion of the radar signalreflected by the person. A processor connected to the radar receiverprocesses the portion of the radar signal detected by the radar receiverto determine whether the person is carrying a concealed object. Thehandheld scanner may include a display and may produce a real-timealert, such as an audible alert, when a concealed object is detected. Inone implementation, the radar signal uses a frequency betweenapproximately 1.5 gigahertz and 12 gigahertz. The radar signal may be afrequency stepped signal, and may have an output power of less than onemilliwatt. In one implementation, the handheld radar frequency scannersystem is a monostatic radar system. In another implementation, thehandheld radar frequency scanner system is a bistatic radar system.Implementations of the transmit antenna and the receive antenna mayinclude a spiral antenna or an endfire waveguide antenna.

Other features will be apparent from the following description,including the drawings, and the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a top view of a handheld radar frequency scanner.

FIG. 1B is a side view of a handheld radar frequency scanner.

FIG. 1C is a block diagram of a handheld radar frequency scanner.

FIG. 2A is a diagram showing concealed object detection using thehandheld radar frequency scanner of FIGS. 1A, 1B, and 1C.

FIGS. 2B and 2C are graphs illustrating signals produced by the systemof FIG. 2A.

FIGS. 3A and 3B are perspective views of an antenna design for thesystem of FIGS. 1A, 1B, and 1C.

FIG. 4 is a diagram of a system for testing concealed object detectionusing the handheld radar frequency scanner of FIGS. 1A, 1B, and 1C.

FIG. 5 is an illustration of objects used for detection in the system ofFIG. 4.

FIG. 6 is a graph illustrating performance of the system of FIG. 4 usingthe objects of FIG. 5.

Like reference symbols in the various drawings indicate like elements.

DESCRIPTION

A handheld scanner transmits a radar frequency (RF) signal from anantenna. The signal is reflected and the reflected signal energy isdetected and processed. The processing methodology used by the handheldscanner determines when the reflected signal appears to be coming fromobjects that need to be detected. The handheld scanner applies detectionalgorithms in the context of aviation security and other situations todetect explosives and other objects on the body of a person, such as aperson passing through a security checkpoint.

In one implementation, the handheld scanner may use detection algorithmsdeveloped for buried mine detection. In the case of non-metallic minesburied in the ground, algorithms developed for mine detection have beenvery successful in the detection of explosives contained within aplastic housing. In the case of concealed objects on a person, such as,for example explosives that have a form factor of one cm thickness overan extended area of several cm length and width, similar detectionmethodology may be used. However, other detection algorithms well knownin the art may also be used.

The detection algorithms detect the reflected body signal. Whenalgorithms developed for mine detection are used, the reflected bodysignal corresponds to detecting the signal reflected from the ground inmine detection. When an object is present on the body, the reflectedsignal amplitude, phase, and frequency content and timingcharacteristics will change. It is this change in the reflected signalthat is recognized by the algorithms and declared as a detection of anobject. The advanced detection algorithms used by the handheld scannerperform with a high probability of detection (Pd) and a low false alarmrate (FAR).

While the handheld scanner is comparable in some ways to a detector usedfor buried mine detection, there are several differences including, forexample, a different frequency band and a different system/antenna size.Also, different detection algorithms may be used. In general, thefrequency band used by the handheld scanner is wider and of slightlyhigher frequency than typically is useful for buried mine detectionbecause, among other things, clothing transmits higher frequencies thanare transmitted by several inches of soil. Due to differences betweenthe handheld scanner and the mine detection equipment, differentdetection algorithms may be used, or different parameters may used forthe detection algorithms in order to optimize system performance for thedetection of objects on a person.

Referring to FIGS. 1A and 1B, a handheld radar frequency scanner 100includes antennas 105 and 107 for transmitting and receiving a radarfrequency (RF) signal. The scanner is a bistatic radar system, in thatthere are separate antennas for transmitting and receiving the RFsignal. In one implementation, antenna 105 is connected to a radartransmitter and transmits an RF signal toward a target, and the antenna107 is connected to a radar receiver and receives a portion of the RFsignal that is reflected by the target. In another implementation,antenna 107 is connected to the radar transmitter and antenna 105 isconnected to the radar receiver. In yet another implementation, thehandheld scanner may be a monostatic radar system that uses a singleantenna to transmit and receive the RF signal. The following discussionassumes that the antenna 105 is the transmitting antenna and the antenna107 is the receiving antenna.

The receiving antenna 107 is coupled to an electronics module 110 thatincludes a signal processing system to process received RF signals fromthe receiving antenna 107. The electronics module 110 also includes atransmitter coupled to the transmitting antenna 105. A handle 115assists the operator in manipulating the scanner 100, and a battery pack120 supplies power to the electronics module 110.

The scanner 100 provides low-cost, real-time, and user-friendly objectdetection using ultra-low power radar (< 1/100 specific absorption rate“SAR,” the standard limit for human exposure). The scanner 100 generatesa single-beam, low-resolution surface reflectivity measurement. As theperson is scanned, a detection algorithm executed by the signalprocessor emits a tone or otherwise indicates an object detection if thecontrast in reflectivity indicates that an object such as, for example,a concealed plastic explosive, a box-cutter, a plastic knife, orcurrency is present. Measurements are attempted over a signal bandwidthfrom approximately 1.5 GHz to approximately 12 GHz. Frequencies up toapproximately 12 GHz, or beyond, may be useful when the objectdimensions are thinner than 1 cm. Objects such as, for example, plasticitems, having thicknesses of 2 mm or less, may be detected using thehigher end of the 12 GHz band of frequencies.

Referring to FIG. 1C, in one implementation, the transmit antenna 105 isconnected to a radar transmitter 125 that transmits an RF signal towarda target. The scanner 100 also includes a receive antenna 107 connectedto a radar receiver 130 that receives the reflected RF signal from thetarget. The receiver 130 is coupled to a signal processing system 135that processes received RF signals from the receiving antenna 107. Thesignal processing system 135 is coupled to a display 140 and a timingand control module 150. The display 140 provides an audible and/or avisual alert when an object is detected by the scanner. The timing andcontrol module 150 may be connected to the transmitter 125, the receiver130, the signal processor 135, and the display 140. The timing andcontrol module provides signals, such as a clock signal and controlsignals, to the other components of the scanner 100. The electronicsmodule 110 of FIGS. 1A and 1B contains the transmitter 125, the receiver130, the signal processing system 135, the display 140, and the timingand control module 150.

FIG. 2A illustrates a system 200A for concealed object detection usingthe handheld scanner 100 of FIGS. 1A, 1B, and 1C. The handheld scanner100 is held at a distance 220 over a clothing layer 205 of a person tobe scanned. The distance 220 may be a fixed or a variable distance, andmay vary within a scan as well as from scan to scan. The clothing layer205 is on top of the body 210 of the person to be scanned. There is anexplosive device or other object 215 concealed between the clothinglayer 205 and the body. For simplicity, the clothing layer 205 is shownas being in direct contact with the body 210, and the object 215 isshown as being in direct contact with the clothing layer 205 and thebody 210. In actual practice, there may be a gap between the body 210and the clothing layer 205, and gaps between the clothing layer 205, theobject 215 and the body 210. However, such gaps do not result in asignificant change in performance because the dielectric constant ofclothing is close to that of air.

The scanner 100 is moved over the clothing layer 205 from a startposition 230 to a finish position 235. The path of the scanner includesan object start position 240 and an object end position 245.

FIGS. 2B and 2C illustrate signals produced by the system of FIG. 2A inresponse to the concealed object 215. In particular, FIG. 2B illustratesan amplitude response signal 250 produced in response to the object 215positioned under the clothing layer 205 and FIG. 2C illustrates a timedelay signal 255 produced in response to the object 215 positioned underthe clothing layer 205. As shown, both of the signals 250 and 255include significant level changes in response to the object 215 as thescanner 100 is scanned from the object start position 240 to the objectend position 245.

The reflection from object 215 differs from that of the body 210 basedon the material dielectric properties. The signal passes through theobject 215, reflects off the body 210, and passes back through theobject 215. As shown in FIGS. 2B and 2C, the portion of the signal thatencounters the object 215 is attenuated and time delayed relative to thereflection from the body 210 alone.

A wideband signal may be used to obtain a time resolution of the bodysurface 210 and the time difference between the signal reflected by thesurface of the body 210 and the signal reflected by the surface of theconcealed object 215. This time resolution allows for thicknessmeasurements of the concealed object 215 and discrimination of unwantedclutter that can occur in the signal return. A wideband signal may alsobe used to mitigate the fluctuations that can occur due to specklesignal cancellation if a single frequency or narrow band of frequenciesis used. Speckle signal cancellation occurs when the complex reflectionsfrom the body 210, the concealed object 215, and other spuriousreflections add up to zero and therefore cancel each other, resulting inno change in the signal level being observed and detection of noindication of the presence of the object 215. By using a wide band offrequencies, the chances that speckle signal cancellation can occur arereduced.

The time-resolved signal reflection from the body 210 of the person isattenuated mainly due to the phase disturbances caused by the presenceof the concealed object 215. The integrated signal energy over theentire band of frequencies under consideration may not change in value,but the phase of that energy versus frequency is modified significantlyso as to be nonlinear. When the time-resolved signal is computed, thenonlinear phasing causes a modified peak signal response from the body210 and a time shift to occur. The signal energy is spread out when thepeak signal is reduced, and occurs at other times in the signal return.

There are many ways to compute the time-resolved signal. For example aFast Fourier Transform (FFT) or a chirp Z transform may be used. Thetime signal may be computed in software, or may be computed inspecialized hardware, such as a Digital Signal Processing (DSP) chip ora specialized compression chip device.

The detection of objects on personnel is indicated by the signalamplitude response. One well known challenge involved with detectionprocesses is to ignore clutter. Clutter consists of unwanted targetdetections, known as false alarms. Many signal processing techniques areavailable to reduce the occurrence of false alarms. In oneimplementation, processing techniques to reduce false alarms in thedetection of landmines may be used.

One exemplary technique to reduce false alarms is known as principalcomponent analysis (PCA). Amplitude and phase response data is availableat chosen frequency steps from the radar. This data is used to computethe PCA response for the non-targets areas, the false target or clutterareas, and the desired targets. A statistical database is then developedfor the non-targets and the targets. The database is then referenced inreal-time by the algorithm, and a detection is declared if the PCAmeasurement characteristics match the characteristics of a target in thedatabase. The actual reference PCA statistical database is also updatedin real-time to account for changes in the environmental conditions asthe scanning is being conducted. Thus, for example, people withdiffering characteristics such as more or less clothing or differentbody parameters will not cause false alarms.

RF energy interacts with the body 210 depending on frequency. Forexample, the body absorbs from 30 to 300 MHz, partially absorbs from 300MHz to 6 GHz, partially or diffusely reflects from 6 GHz to 15 GHz, andhighly/specularly reflects at >15 GHz. Use of frequencies in thehighly-reflective region Ka-band (27 to 33 GHz) may produce specularreflection images, which may cause difficulty with the detection of theconcealed object 215. Ku-band (12 to 18 GHz) may produce better results,but straddles the transition from diffuse to specular reflection atapproximately 15 GHz.

The techniques may employ the absorption/reflection transition frequencyregion from approximately 1.5 GHz to approximately 12 GHz. In thisregion, specific frequencies that maintain high dielectric differencesbetween explosives and the body surface coexist with diffuse reflectionfrom the skin and shallow body depths. This maintains radar crosssection or reflection of the target without spoofing the detectionprocess through specular reflection above approximately 12 GHz andwithout driving the radar cross section to unacceptably low levels belowapproximately 1 GHz.

In the region from 1.5 to 12 GHz, the dielectric for explosives is near3, while the body covers ranges of 5 for fat, 10 for bone, 40 for skin,and 50 for muscle. This compares with the mine detection problem, wherean explosives dielectric near 3 may be embedded in dielectrics between 4for sand and 20 for wet loam. For concealed object detection, theclothing dielectric is near 1.1 (with air at 1.0) and therefore providesno significant reflection or effect on the RF energy. The higherdielectric is behind the target object, and there is typically muchgreater difference between the explosive dielectric and the body (forexample, explosive to sand is a difference of 1; explosive to skin is adifference of 37).

Allowable general population/uncontrolled exposure sets the toughestlimit on human exposure to RF radiation. At the frequencies of interest,the allowable density is 1 mw/cm2 (FCC) for 30 minutes averaging time,or about ⅕ to 1/10 of controlled exposure limits (IEEE and OSHA) for 6minutes averaging time. At the surface of the transmitter, a density of0.1 mW/cm2 is typical. Exposure while scanning the body is for less than1 minute or about 1/30 the allowed time period, which in combinationwith the 1/10 lower maximum energy density, yields a very safe 1/300exposure margin. By comparison, 600 mW is allowed for cell phones suchthat one could place his or her head against a transmitter at maximumpower of 10 mW and still be exposed only to one sixtieth of thepermitted radiation for cell phones.

The handheld scanner 100 is also below the specific absorption rate(SAR) limit of 0.4 W/kg. Scanning of the entire body (typically 1.75 m²surface area) yields 0.035 W/kg for a typical 50 kg individual. This is11 times less than required limits.

In one implementation, the RF models that have been developed, updated,and correlated with mine radar background and target characteristics areextended to the aviation security scenarios through use of the describedhandheld scanner. The models include target cross-section, SNR, andalgorithm receiver operating characteristics (ROC) curves.

To establish the models, radar data is collected against desired targetsand segregated into algorithm training and testing groups. Typically,enough data is collected to separate the groups wholly (to demonstratethe required Pd and FAR at 90% confidence, for example). Alternatively,so-called ‘leave-one-out’ testing may be performed to achieve nearly thesame statistical significance with half the data. Training and testingon the same data (Train A/Test A) only provides a lower bound on theBayes error for the population distribution. Train A/Test B (orequivalently, leave-one-out training/testing) provides the upper boundon the Bayes error that is practically a more useful predictor of systemperformance.

System performance may be defined in view of Pd versus FAR. Both Pd andFAR are evaluated together, preferably using ROC curve methodologiesincluding confidence intervals. There are two measures of FAR, perindividual and per group. For the former case, using 1.75 m² for surfacearea, and assuming similar system performance to the mine detectionperformance, then scaling this performance along the ROC curve yields a2% FAR per individual at the required 90% Pd. Typically, only 1 in 50individuals would have to be searched after using the handheld scanner.

For the aviation security problem, however, significantly betterperformance (near 100% Pd and near 0 FAR) is expected because of therelatively controlled/benign environmental factors involved, versus theunconstrained, outdoor, small mine detection problem. Specifically, themine detection technology was designed and successfully tested againstthree types of backgrounds: gravel, grass, and bare soil. These terrainsincluded, for example, buried roots and rocks. The targets included a 6cm circular plastic encased explosive that is significantly smaller thana baseline 10 cm by 7.6 cm ‘wallet’ target.

Use of radar technology permits the system to provide real-time alerts.Typically, a real-time alert is in the form of an audio signal, so thatlittle operator training and no operator interpretation is required.Typically, no actual imagery of either the object itself or the subjectbody will be created or displayed.

Real-time audio detection may be made available using multi-threadedprocessing that permits the simultaneous operation of advanced AutomaticTarget Recognition (ATR) algorithms, the Ground Penetrating Radar (GPR)control code, system tasks, and other tasks. The processing of GPR datamay rely on Principal Component Analysis (PCA) performed automaticallyupon appropriately selected and conditioned features of the GPR responsein clutter and target environments.

A frequency-stepped radar permits operation at an RF duty factorapproaching unity. This removes the short-pulse radar requirement thatthe RF equipment (transmitter, antenna, and receiver) be instantaneouslybroadband. It also achieves a fully coherent radar capability whileretaining (and expanding) the achievable high-range resolutioncapability. A high RF duty factor allows the thermal-noise-limiteddetection sensitivity of the radar to be achieved using readilyavailable components. With the high RF duty-factor-stepped frequencywaveform, the sensitivity limit is typically dictated by parametersrelated to the environment in which objects are located. Thefrequency-stepped radar is fully coherent, which allows for compensationof hardware amplitude and phase (dispersion) errors over the operatingRF band.

A bi-static system (separate transmit and receive antennas) is used withappropriate calibration and signal processing. The described systemallows for bandwidth control, a resultant sensitivity increase and anantenna match that can be achieved given the frequency bandwidth thatmust be covered and the various conditions through which the antennamust propagate energy. The overall radar specifications for oneimplementation are provided below in Table 1.

TABLE 1 Penetrating Radar Parameters Radar type Frequency steppedReceiver sensitivity −110 dBm Antennas spirals or end fire waveguideFrequency band 1.5 to 12 GHz Power output transmit <1 milliwatt Power 24V dc, 4 amps Electronics/system weight 2 kg

Many types of antennas can be used for the handheld scanner, includingspiral antennas and endfire waveguides. The requirements for the antennaare that they be able to transmit and receive the selected signalbandwidth without significant degradation of the amplitude and/or phaseof the signal. In practice, the size of the antennas determines the sizeof the scanner search face. Thus, smaller antennas are more practicalfor the handheld scanner.

FIG. 3A illustrates an antenna design 300 employed in one implementationof the handheld scanner of FIGS. 1A, 1B, and 1C. The design 300 employsseparate transmit 305 and receive 310 antennas to simplify theelectronics, provide spatial separation and reduce very shallowreflections. The antennas 305 and 310 may be placed in a housing 315,and a cover 320 may be placed over the antennas. The cover 320 is madeof a suitable radome material.

FIG. 3B further illustrates aspects of the antenna design 300 discussedabove with respect to FIG. 3A. Although the following discussion refersto receive antenna 310, it is equally applicable to transmit antenna305. As shown, the design 300 employs a spiral antenna 310 that isattractive from the viewpoint of size reduction. For an antenna to be anefficient radiator, it must normally have a dimension of at leastone-half wavelength. The spiral radiates efficiently when it has anouter circumference of at least one wavelength. This means that theantenna needs a maximum diameter of about one-third wavelength. Theupper frequency limit for efficient spiral radiation is set by the sizeof the feed point attachments, and the lower frequency limit is set bythe outer diameter of the spiral structure. Within these limits, thespiral radiates efficiently in a frequency-independent manner. The inputimpedance and the radiation patterns will vary little over thisfrequency range. With a spiral structure, an upper frequency of 12gigahertz presents no problem. The spiral antenna produces acircularly-polarized signal.

The spiral antenna 310 is constructed by etching the pattern on aprinted circuit board. A planar, printed circuit, spiral antennaradiates perpendicularly to the plane of the spiral. The spiral 325itself is located at the end of a cylindrical metal cavity 330 (thecavity back) to provide isolation from neighboring elements andelectronics. Typically, an absorber 335 is used on the top side of thespiral inside the cavity 330 to make sure the element responds onlydownward.

Another type of antenna which may used is an endfire waveguide antenna.The configuration is slightly larger than the spiral configuration. Theendfire waveguide antenna reduces the measurement spot size, thus makingthe exact position of the concealed object detected easier to locate.Other suitable types of wideband antennas may also be used FIG. 4 showsa system 400 for testing concealed object detection using the handheldradar frequency scanner of FIGS. 1A, 1B, and 1C. The handheld scanner100 is passed over an object 410 placed in front of a simulated body.The body reflection is simulated by water 405 placed in a containerwhich provides a reflection surface similar to that of a person'sstomach.

FIG. 5 shows objects 500 which may be used as examples of the object 410described with respect to the system of FIG. 4. The objects 500 includea plastic knife 505, a plastic ruler 510, and a metal ruler 515.

FIG. 6 illustrates the amplitude response of the signal return whenobjects 500 described with respect to FIG. 5 are used as the object 410in the system of FIG. 4. In particular, signal 605 is the waterinterface simulating the body reflection. Signals 610, 615, and 620 are,respectively, the signals corresponding to the use of the plastic knife505, the plastic ruler 510, and the metal ruler 515 as the object 410 inthe system 400 of FIG. 4. As shown, there is a significant signalreduction when any of the plastic knife 505, the plastic ruler 510, andthe metal ruler 515 are encountered.

Suitable processing hardware for the handheld scanner includes anydigital-based high speed processing equipment.

The described systems and techniques provide low cost, ease of use, andhigher performance (better detection and lower false alarm rate) for ahandheld scanner. A low-cost, user-friendly handheld scanner fordifficult concealed object detection employs a comprehensive antenna andRF module design, along with sophisticated concealed/buried targetdetection algorithms. Data collected against different targets and indifferent conditions, together with adaptive/advanced algorithms, enableprediction and validation of performance for a range of potentialoperating frequencies.

The implementation of the handheld scanner typically is mechanicallysmall so that the scanning operation can be easily accomplished. Also,the signal generation and reception electronics and the processingelectronics typically are all contained within the handheld scanner.However, some components may be external to the scanner. The devicepower may be a battery or an AC power cord. The scanning operationconsists of moving the antenna head part of the handheld device over theperson being scanned, typically maintaining the distance ofapproximately 0–3 inches clearance. Touching the person with the deviceis not necessary. The sensitivity of the device may be reduced as theclearance increases beyond approximately three inches.

Other implementations are within the scope of the following claims.

1. A handheld radar frequency scanner system for detecting objectsconcealed on a person, the system comprising: a radar transmittercoupled to a transmit antenna that is configured and positioned todirect a radar signal at a person; a radar receiver coupled to a receiveantenna that is configured to detect a portion of the radar signalreflected by the person; and a processor connected to the radar receiverand operable to process the portion of the radar signal detected by theradar receiver to determine whether the person is carrying a concealedobject, by conducting a test in which a first characteristic of a firstdielectric constant associated with the person is determined, and asecond characteristic of a second dielectric constant associated withthe concealed object is determined.
 2. The system of claim 1 wherein thesystem is configured to produce a real-time alert when a concealedobject is detected.
 3. The system of claim 2 wherein the real-time alertcomprises an audible alert.
 4. The system of claim 1 wherein the radarsignal comprises a frequency between approximately 1.5 gigahertz and 12gigahertz.
 5. The system of claim 4 wherein the radar signal comprises afrequency stepped signal.
 6. The system of claim 4 wherein the radarsignal comprises an output power of less than one milliwatt.
 7. Thesystem of claim 1 wherein the handheld radar frequency scanner systemcomprises a monostatic radar system.
 8. The system of claim 1 whereinthe handheld radar frequency scanner system comprises a bistatic radarsystem.
 9. The system of claim 1 wherein at least one of the transmitantenna and the receive antenna comprises a spiral antenna.
 10. Thesystem of claim 1 wherein at least one of the transmit antenna and thereceive antenna comprises an endfire waveguide antenna.
 11. The systemof claim 1 wherein the first dielectric constant is determined duringthe test.
 12. The system of claim 11 wherein the first dielectricconstant is associated with skin of the person.
 13. The system of claim11 wherein the second dielectric constant is determined during the test.14. The system of claim 5 wherein the processor is operable to conductthe test using a frequency-stepped scheme.
 15. The system of claim 5wherein the frequency stepped signal comprises frequencies at definedintervals throughout a defined frequency range.
 16. The system of claim1 wherein the processor is operable to conduct the test based upon adifference between the first dielectric constant and the seconddielectric constant, wherein the difference is determined during thetest.
 17. The system of claim 1 wherein the processor is operable todetermine whether the person is carrying a concealed object using aprincipal component analysis technique.
 18. A handheld radar frequencyscanner system for detecting objects concealed on a person, the systemcomprising: means for directing a radar signal at a person; means fordetecting a portion of the radar signal reflected by the person; andmeans for processing the portion of the radar signal detected by theradar receiver to determine whether the person is carrying a concealedobject, by conducting a test in which a first characteristic of a firstdielectric constant associated with the person is determined, and asecond characteristic of a second dielectric constant associated withthe concealed object is determined.
 19. The system of claim 18 furthercomprising means for producing a real-time alert when a concealed objectis detected.
 20. The system of claim 19 wherein the real-time alertcomprises an audible alert.
 21. The system of claim 18 wherein the firstdielectric constant is determined during the test.
 22. The system ofclaim 21 wherein the first dielectric constant is associated with skinof the person.
 23. The system of claim 21 wherein the second dielectricconstant is determined during the test.
 24. The system of claim 18wherein the radar signal comprises a frequency stepped signal andwherein the test is conducted using a frequency-stepped scheme.
 25. Thesystem of claim 24 wherein the frequency stepped signal comprisesfrequencies at defined intervals throughout a defined frequency range.26. The system of claim 18 wherein the means for processing is operableto conduct the test based upon a difference between the first dielectricconstant and the second dielectric constant, wherein the difference isdetermined during the test.
 27. The system of claim 18 wherein the meansfor processing is operable to determine whether the person is carrying aconcealed object using a principal component analysis technique.
 28. Amethod for detecting objects concealed on a person using a handheldradar frequency scanner system, the method comprising: directing a radarsignal at a person; detecting a portion of the radar signal reflected bythe person; and processing the portion of the radar signal detected bythe radar receiver to determine whether the person is carrying aconcealed object, by conducting a test in which a first characteristicof a first dielectric constant associated with the person is determined,and a second characteristic of a second dielectric constant associatedwith the concealed object is determined.
 29. The method of claim 28further comprising producing a real-time alert when a concealed objectis detected.
 30. The method of claim 29 wherein the real-time alertcomprises an audible alert.
 31. The method of claim 28 whereinprocessing further comprises determining the first dielectric constant.32. The method of claim 28 wherein processing further comprisescomparing the first dielectric constant and the second dielectricconstant.
 33. The method of claim 28 wherein processing furthercomprises: determining the first dielectric constant; determining thesecond dielectric constant; and determining that a defined discrepancyexists between the first dielectric constant and the second dielectricconstant.